For many years, computer printers had the capability of printing on only one side of paper. Recently, however, printing on both sides of paper has become the norm. With continuous forms (e.g., roll-fed) printers, two-sided printing is typically accomplished by using a pair of print engines, with a mechanism in between them that turns the paper over as it travels between print engines. However, there are a number of technical challenges for a printing system in carrying out such a procedure reliably and accurately.
Moreover, it would result in an extremely serious error if the front and back sides of a sheet were ever printed with the mismatched sides. Such an error is referred to as a side mismatch. A side mismatch may result in, for example, a hospital bill being printed with the front side having the name and address of patient A, and the back side having the itemized invoice details for patient B.
As a mechanism for detecting side mismatches, two-engine duplex printers often associate a unique identifier with the data for a pair of matching sides, and then print a machine-readable version of the identifier on the first side of the sheet and then read that mark in the second printer, and compare the value read with the value just printed or about to be printed by the second printer. If the value read is not as expected, that means a side mismatch error has occurred (or is about to occur). This system of printing, reading, and checking unique identifiers to prevent side mismatch errors is known as a Side 2 Verification system.
Another mechanism used to detect side mismatches implements industry standard barcodes that are printed on both the front and the back of each sheet, where the same barcode value is written on both sides of a sheet and each subsequent sheet gets a new barcode value. Two barcode readers are positioned to read the barcodes on each side of a sheet. Subsequently, the read values are compared. If the values are not identical for a given sheet, then there is a side mismatch error.
A problem exists with using such a mechanism on a color printer since the color printed on a page may come from a combination of several planes of color (e.g., cyan, magenta, yellow, and black), and the data for each of these colors is processed through a series of parallel but separate components. Thus, the potential exists for a side mismatch error to happen on only one color plane. Because barcodes are conventionally printed only in black, the potential exists for a side mismatch error to escape undetected when the error does not affect the black color plane.
Accordingly, a mechanism to detect a side mismatch error that occurs in one or more color planes is desired.